In typical magnetic recording systems, data is recorded onto a magnetic medium, such as a tape or a disc, by means of a magnetic recording head housed in a drive. The data is recorded onto the medium in tracks, whose width and spacing determine the density, and thus the amount, of the data which can be recorded onto the medium. The head "reads" data signals from the recording medium having a particular acceptable range of orientation, and disregards signals having an orientation which is sufficiently oblique or angled relative the coherent data signals.
Problems with reading the data recorded on the medium can occur when the drive is subjected to external forces, such as mechanical vibrations resulting from jarring or jolting the drive. In addition, thermal variations within the drive, such as the temperature rise which accompanies warmup, and mechanical and electrical resonances produced by internal mechanisms and circuitry, also produce vibrations within the drive. When such a vibration occurs, the head moves off track and can move partially over other tracks, thereby reading mixed data from the two tracks. In addition, because most drives do not erase unwanted data, but instead, overwrite the existing data with new data, the problem is intensified when the heads write new data over existing data. As will be appreciated, if the recording head is offtrack when overwriting the old data and is later repositioned ontrack to read back the new data, what the head in actuality reads back is a composite of old data signals and new data signals. Since the proper signal on the old data track is the new data signal, the old data signal is considered "coherent noise." Coherent noise is defined as unwanted signals within the bandwidth of the recording code, and is differentiated from broadband thermal noise. Said coherent noise is typically locked in a coherent phase with respect to the desired data signals. If this noise were sufficiently angled from the orientation of the new data signals, it would be disregarded by the head. Unfortunately, all too often the improperly written data is of an orientation similar to that of the desired data track and is mistakenly identified by the head as the desired data signal, or modulates the timing of the desired data signal.
Naturally, vibration during the read operation causes similar problems. That is, even if the data is written on the proper track, vibration will often cause the head to drift partially offtrack during the read operation. Although data is generally written with sufficient density to be read by the head even if the head is partially offtrack, head performance will fall off dramatically if the head picks up sufficient coherent noise from the portions of the recording medium surrounding the desired track. Depending upon track density and the amount of head drift, the coherent noise could include the data from the edge of the adjacent track.
To prevent the head from reading data from two adjacent tracks, areas of the medium, known as guardbands, are commonly positioned between the tracks. Essentially, the guardbands are "strips" of the medium located between information tracks where no data is written. Thus, if the head moves off the desired track during the read operation, no other data from adjacent tracks will be read, and the amount of coherent noise experienced by the head will be drastically reduced.
Guardbands are limited in their capacity to compensate in disc drive failure, however, by their inability to correct for data which has been initially written offtrack. When mechanical vibrations occur which cause the head to move offtrack during the write process, data will be written partially onto the track, as well as onto a portion of the guardband adjacent the track. Later, during the read operation, vibrational head drift will cause the head to read data from both the track and the adjacent guardbands. When the medium is only used for a single write operation, the only coherent data on the guardbands will be the data recorded on the track. When the medium is written over numerous times, the random head drift will cause the guardbands to be filled with a nonsensical combination of misplaced writes and this coherent noise will severely compromise head performance.
To overcome this problem, bands on either side of the information tracks can be erased with each write operation. Because the erase bands are created with each write operation, the bands do not become filled up with misplaced writes and head performance is improved. Some prior art methods of forming erase bands consisted of erasing the data on either side of an information track and then recording data along the track in a separate operation. U.S. Pat. No. 4,644,421 and U.S. Pat. No. 4,290,088 are examples of such methods. While these methods provide for erase bands between adjacent tracks, the process is time consuming and requires multiple operations to achieve erasure.
Magnetic heads have been developed which create erase bands between tracks on the information medium as data is written onto the medium. U.S. Pat. No. 3,155,949 discloses a magnetic transducer assembly comprising a read/write element and an erase element located immediately downstream of the read/write element. The read/write gap and erase gap are of equal widths. However, the erase gap is interrupted by a tunnel or notch. The erase element is activated during the write operation to erase both lateral edges of the written information. This design, however, requires precise and separate machining of the tunnel at increased manufacturing costs so that the desired recording and track widths are obtained.
In the IEEE publication "THIN FILM FLOPPY DISK HEAD", Chang, et al. disclose a magnetic head in which straddle erase of data tracks on a floppy disk is accomplished using the same coil layers as those of the read/write head. As two main pole pieces of the head write data onto the medium, separate erasure poles located on both sides of the write poles write a magnetization pattern which is perpendicular to the direction of the data magnetization. However, this design is also expensive to manufacture due to the extra maskings required to produce the head structure. Further, the width of the erase bands created by this head structure is limited by the width of the gaps between the main write poles and erasure poles.